admin configuration: don't show any email server config options (e.g., smtp or sendgrid) if email sending isn't enabled at all anyways

[williamstein]

Justification: it's very confusing to see e.g., an option to select smtp, but no way to configure it. This is what happens if you have email disabled. Screenshot:

In contract, if you enable email then things make sense:

This sufficiently confused a person just now that they opened a support request...

[williamstein]

An extra reason for confusion is:

The instructions say

If 'default', it uses the usual email backend to send password resets so I was assuming the settings under the main Email backend type would apply.

And that was true until very recently -- we only just added support for using smtp for everything, rather than requiring sendgrid for some emails.

So we should also change that wording, or clarify it.